Method of coating surfaces with boron



Patented Oct. 11, 1949 y f2,4s4;519,; METHOD OF COATING SURFACES WITHORON Graham Robert Martin, Montreal, Quebec, Canada No Drawing.Application July 26,

1946, Serial N0.

686,453. In Canada January 15, 1946 4 Claims. 1

This invention relates to the coating of various surfaces with boron andmore particularly to a method of forming a thin film containing boron onmetallic, glass or other surfaces.

Because boron under the influence of fluxes of thermal neutrons emitscharged particles the ionization due to which can be measured, it hasbeen used in ionization chambers employed in detecting neutrons and inmeasuring their intensities. Previous methods have employed an adhesivevehicle containing a boron compound, such as boron carbide, in finelydivided condition. Boron obtained by the thermal decomposition ofdiborane has been used but diborane is difficult to obtain. Other usesof surfaces coated with a thin film of boron will suggest themselves tothose skilled in the art.

The object of the present invention is to provide a method wherebyreadily available boron compounds may be employed to deposit from thevapour phase a continuous film containin boron uniformly distributed ona surface to be coated.

In carrying out the invention boron alkyl compounds are used to providethe boron. Boron trimethyl is preferred. The triethyl and the tripropylcompound may be used but at somewhat lower temperatures. Boron trimethylis conveniently prepared in accordance with the following equations:

An ethereal medium is used, and, for convenience in handling the borontrifiuoride is added in the form of its ether adduct, (C2H5)2OZBF3.Since the product is spontaneously inflammable, the reaction is carriedout in an atmosphere of an inert gas, such as nitrogen, and the borontrimethyl is separated by condensation in a trap cooled by liquidnitrogen. It is purified by fractional distillation at 95 C. and isstored as a gas. It boils at 20.2 C.

In forming the coating the article to be coated is placed in an oven inwhich it may be heated to the desired temperature of about 550 C. andthe gaseous boron alkyl compound is passed into the oven under apressure of about 1 cm. of mercury to contact the article. Provision isnecessarily made to evacuate the oven before admitting the gaseouscompound. The temperature of the article may be maintained by anysuitable furnace electrically or otherwise heated. The boron alkylcompound is thus thermally decomposed at and on the heated surface ofthe article yielding free boron which deposits adherently on the 2surface. A portion of carbon usually deposits along with the boron, butfor use in ionization chambers this is of little consequence.

For coating a steel surface of 1000 sq. cm. about 8 liters of gaseousboron trimethyl passed over the surface, maintained at a temperature ofabout 550 C., during a period of about two to three hours gave a goodcoating.

When the surface to be coated is relatively small in relation to thevolume of the furnace, it has been found advantageous to add the boronalkyl compoimd to the oven intermittently. Thus the compound may bereleased under reduced pressure into the oven where the pressure mayreach about 2 cm. of mercury and allowed to react for a short periodwhen the residue of gases is pumped off to a pressure of say 0.1 mm. andthen more of the boron alkyl compound is admitted. This is repeateduntil the desired thickness of coating is obtained. With this procedurethe amount of the boron compound required for a given coating isreduced.

These ionization chambers are useful for measuring fluxes of slowneutrons. For ionization chambers for the monitoring and control ofpiles a coating of about 1 mg. of boron per square centimeter issufficient. The nuclear reaction involved is:

The range of alpha particles from the reaction corresponds to a slightlysmaller thickness than this, and the slow neutron flux is reduced to theextent of about 4% per mg./sq. cm. of coating, with a consequentreduction in the ionization current produced.

It will be apparent that various forms of apparatus may be employed forcoating surfaces by the method described.

I claim:

1. A method of coating surfaces which comprises maintaining the articleto be coated at a temperature of about 550 C. in an evacuated chamberand passing one of a group consisting of trimethyl boron, triethyl boronand tripropyl boron in the vapour phase'and under reduced pressure intocontact with the surface of the heated article to cause boron to bedeposited thereon in a thin film.

2. A method as defined in claim 1 wherein said compound is admitted tothe chamber intermittently.

3. A method as defined in claim 1 wherein said compound is continuouslyadmitted to the chamber.

4. A method of coating surfaces of articles with REFERENCES CITED boronin a chamber which comprises evacuating the chamber, heating the articlein the chamber The following references are of record in the to atemperature of about 550 C. and admitting file of this patent: to thechamber one of a group consisting of tri- 5 methyl boron, triethyl boronand tripropyl boron UNITED STATES PATENTS to a pressure of about onecentimeter of mercury Number Name Date to contact the surfaces of theheated article and 597,172 WilmOWSkY 11, 1893 deposit boron thereon.1,774,410 Van Arkel Aug. 26, 1930 GRAHAM ROBERT MARTIN. H 10 '2.303,658Porter Dec. 1, 1942

